1. Field of the Invention
The invention relates to a coupled multi-band antenna and, in particular, to a multi-band antenna with the broadband function.
2. Description of Related Art
Personal mobile communications have proved its great potential and business opportunities in radio communication industry. During its evolution process, many systems have been developed, using different technologies and channels. They also play important roles in different areas and markets. However, such varieties cause troubles and inconvenience for both system suppliers and consumers. One crucial point is that different systems use different frequency bands (e.g., GSM900, PCS1900, and UMTS).
In order for users to operate with greater ease, the industry has invested a lot of manpower to develop products with multi-band integrations. However, the first difficulty to overcome is the antenna. One may say that the antenna is both the beginning and end of the wireless communications. Its properties directly affect the communication quality. An antenna has to satisfy the following requirements:
1. wide frequency and bandwidth; and
2. good matching between antenna radiation efficiency and field.
A recent trend in the design of electronic products is light and compact devices. This directly constrain the size of antenna inside the mobile communication products. Since the planar inverted-F antenna (PIFA) has an operating length of only ¼ wavelength, it is widely used in the designs of hidden small antennas. An example of the PIFA working at a single frequency in the prior art is given in U.S. Pat. No. 5,764,190. Afterwards, in order for the PIFA to work in multi-bands, the radiating elements are suggested to have L-shaped or U-shaped holes.
Another antenna that can achieve multi-band operations is illustrated in FIG. 1. The antenna has a first radiating part A, a second radiating part B, and a ground part C. The first radiating part A and the second radiating part B extend from the two opposite sides of one end of the ground part C. The first radiating part A includes a first conductive plate A1 and a first connecting part A2 connecting the first conductive plate A1 and the ground part C. The second radiating part B includes a second conductive plate B1 parallel to the ground part C and a second connecting part B2 connecting the second conductive plate B1 and the ground part C. The first conductive plate A1 and the second conductive plate B1 extend respectively from the first connecting part A2 and the second connecting part B2 toward the same direction.
Although the above-mentioned antenna can achieve multi-band operations, it nevertheless has the following drawbacks. The distance between the first conductive plate A1 and the second conductive plate B1 is too short, resulting in insufficient bandwidths in both high and low frequencies. Moreover, the small distance also causes large production errors in practice, lowering the yield. At the same time, the feed wire and the feed point are close to the first connecting part A2. Therefore, the bandwidth of the conventional PIFA has an upper limit, unable to achieve broadband effects.
To solve the above-mentioned problems, the invention proposes a novel design of coupled multi-band antenna with the broadband function. The disclosed antenna utilizes a coupled radiator to feed electrical signals into the antenna radiator in a coupled method. It avoids the drawback of a limited bandwidth in the conventional PIFA and reaches the goal of multi-band operations. Using two radiating extensions, the surface current distribution and impedance variation of the antenna can be effectively controlled to achieve broadband and higher radiation efficiency. Consequently, in addition to a novel structure, the disclosed antenna great enhances the bandwidth and efficiency and includes multiple system bands.